"sck0006" wrote in message
...
I have a Kenwood VR-407 that keeps going into protection (turns off &
power light blinks). The owner stated that it was a sudden
occurrence, no lightning strikes, shorted wires, etc... This unit uses
two amplifier modules, one STK413-020A for front & center channels, and a
STK412-020A for the surrounds. I removed both odules, the unit powered
up no protection. I put in the front module, he unit powered up no
protection. I removed the front & put in the ear, the unit went into
protection. I assumed the rear module was lown, purchased he new one for
~$30.00, put it in, & the unit still oes into rotection. I don't have the
service manual on this unit, but rom hat I can tell, each module uses the
same four power supplies. It
looks like there's a ±65V & a ±35V. Both supplies come up w/ no
ripple before the unit goes into protection. I wasn't able to get any
sort of quantitative measurements from the output terminals (at the
amplifier outputs, of course the rear panel terminals are cut off
because of the speaker relays), but I can safely say that none of the
output terminals are shorted to any supply or ground. I checked all
thru hole semi's for a nominal pn drop, nothing shorted out there,
which brings up another point.

How does everyone check transistors/diodes in-circuit? I've heard
many times that just checking the pn junction voltage isn't enough,
and I'll admit I've had a few that had a proper voltage but were
actually bad, but for the most part shouldn't it suffice?

I'm sure this will probably open up a battle between greats, but I'd
love to hear from everyone's experiences on this matter.

Thanks,
Steve


Look at the output of each of the rear channels with a scope at the moment
the protection circuit "should" unlock and let the unit play. Maybe a large
DC spike at that moment. If so, you may well have a bad muting transistor.
Used to be a fairly common problem on many brands, though I haven't seen one
just lately...

Muting transistors are somewhat special and subs generally won't work. They
are specially doped so that the large negative bias applied to turn them off
won't damage the base-emitter junction. Sometimes digital transistors are
used for the purpose.

It is possible to defeat the power relay on many models, and bring the unit
up on a variac. This will usually allow you to check internal voltages,
regulated supplies, offsets, etc without regard to protection status.

If the unit has 2 switched outlets, this is easy. Bring the unit's own power
cord to one of it's own switched outlets, and use a double-ended POLARIZED
cord to connect the other switched outlet to your variac (if you have
one...) or a switched outlet strip. Make sure the polarity of the AC cord
has not been compromised or defeated - that hot and neutral don't get
reversed in the process, or there will be major sparks!

You're going to need a service manual if you don't already have one...

Mark Z.